1. Field of the Invention
The present invention relates to video signal processing, and, in particular, to filtering decompressed digital video signals to reduce noise, such as mosquito noise.
2. Description of the Related Art
Mosquito noise is a term used to describe temporal Gibb's phenomena in decompressed digital video. Such phenomena are typically medium-frequency spatial artifacts with a high temporal frequency resulting from truncation and/or quantization of transform coefficients in the compressed digital video representation. Mosquito noise is so termed because it looks like beating mosquito wings, i.e., semi-transparent high-frequency noise, which is usually most apparent on flat areas adjacent to strong edges in the video. One way to reduce mosquito noise is to apply a coring filter using a spatial (e.g., one-dimensional, horizontal) bandpass filter having low-frequency and high-frequency stopbands separated by a single, mid-frequency passband. In such a coring filter, the bandpass filter is applied to one copy of the input signal, and the bandpass filter output is then subtracted from another copy of the input signal to generate a coring filter output having attenuated frequency components corresponding to the passband of the bandpass filter. As used in this specification, “stopband” and “passband” are relative terms, whereby a stopband is a frequency range in which the filter attenuates received signals more than the attenuation applied by the filter to a different frequency range corresponding to a passband.
FIG. 1 shows the frequency response of an exemplary symmetrical 13-tap bandpass filter that can be used in a coring filter to reduce mosquito noise in decompressed digital video. The coefficients [coef 0, coef 1, coef 2, coef 3, coef 4, coef 5, coef 6] of the symmetrical 13-tap bandpass filter of FIG. 1 are [1207, 0, −592, 0, −146, 0, 80]/2048, where coef 0 is the coefficient for the center (i.e., 7th) tap, coef 1 is the coefficient for the 6th and 8th taps, coef 2 is the coefficient for the 5th and 9th taps, and so on until coef 6 is the coefficient for the 1st and 13th taps.
The bandpass filter of FIG. 1 provides a suitable level of noise reduction in some digital video applications. One application in which the bandpass filter of FIG. 1 might not provide an acceptable level of noise reduction is when the digital video signal is upscaled before mosquito noise reduction (MNR) is implemented using the bandpass filter of FIG. 1, such as when standard-definition (SD) video is upscaled to generate high-definition (HD) video. For example, when converting SD video having a width of 720 pixels into HD video having a width of 1280 or 1920 pixels, the SD video is upscaled by a factor of 1.78 or 2.67, respectively, to generate the HD video. In such applications, unacceptable levels of aliasing noise resulting from the upscaling process may be present in the output video signal produced using the bandpass filter of FIG. 1.